1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus such as a copying machine, a facsimile machine, a printer, or other similar image forming apparatus.
2. Discussion of the Background
In an image forming apparatus, an apparatus detecting whether a conveyed medium includes one or more sheets is described in Japanese Patent Laid-Open No. 2000-34037. However, because this apparatus detects whether the conveyed medium includes one or more sheets, even if this apparatus is applied to a paper feeding apparatus, the actual number of the recording paper sheets cannot be detected precisely if several sheets of recording paper are on a paper tray.
Further, another background paper feed apparatus detects an approximate number of paper sheets remaining on a paper tray, for example whether there are 50 or 100 recording paper sheets on a paper tray. However, there is a problem that an inconvenience arises from the inherent inaccuracy and roughness of only approximately detecting the number of paper sheets.
Recently, and as shown in FIG. 16, an image forming apparatus such as a copier or a printer has included a large transfer belt 91 that simultaneously carries plural images G1–G5 (for a total of carrying 5 images) so that the apparatus can have a high speed operation. However, if the number of recording paper sheets on a bottom board 96 of a paper tray 95 is less than the number of images carried on the transfer belt 91 at one time, unnecessary images end up being formed on the transfer belt 91. Therefore, a waste of a toner results because the toner from the unnecessary images is not used to form images, but is only collected by a cleaning device.
For example, if there were only three recording paper sheets (P1–P3) left between the paper tray 95 and the transfer member 94 when the transfer belt 91 carried 5 images (G1–G5) as shown in FIG. 16, two images would be merely erased by a cleaning device because no recording paper sheets would be available to receive those two images. Therefore, as recognized by the present inventors, there is a need for the number of sheets to be detected precisely.
However, there are problems that a number of sheets cannot be optically detected precisely because different kinds of recording paper sheets have different transmitted light rates.
A transmitted light rate of an ordinary recording paper sheet may be equal to or less than 1.5%, but the transmitted light rate may be less than 0.1% when in particular the recording paper sheets include pieces of cardboard. This factor deteriorates measurement precision so that noise influences become large.
If a quantity of emission of a light emitting device is increased in such a case as noted above, the transmitted light rate cannot have a measurement of around 90% for, for example, an OHP (overhead projector) sheet.